PHP 5 - Variable scope across include files with no classes or functions

I've read through many, many threads on this and still can't wrap my head around it.

Here's my basic issue:

header.php includes a file called navigation.php. Within navigation.php, $previous and $next are defined. Using echo statements I have verified they have values.

Next, header.php includes a file called backnext.php. I need backnext.php to know the values of $previous and $next. If I declare them as global at the top of backnext.php, I don't get errors, but echo statements show they are empty. If I don't, I get an undefined variable error.

Where exactly do I need to declare them as global to have backnext.php be able to read their values correctly?

None of these files are using functions or classes.